In such a type of drive, it is difficult to regulate the action of the jack so as to enable the various synchronization steps to take place correctly.
Various systems have been proposed for controlling the force applied to the drive by the jack.
A first solution consists in using a jet in the supply to the jack. This results in slowing down the operation too much, so that this solution is not advantageous. Furthermore, the jet is apt to become clogged.
If the jack is a double action jack, the back pressure generated by the braking action of the fluid escaping from the unutilized chamber causes the working time of the synchromesh device to be reduced, while the total gear change time remains unchanged.
A second solution consists in mounting a pressure reducer in the supply to the jack. This inproves the working conditions for the synchromesh device, but makes the system sensitive to variations in the viscosity of the lubricating oil, owing to the reduction of the force applied to the control, and, for the same reason, prevents the clutch from working safely and totally.
Owing to the poor results achieved, the above expedients have not been very successful. The main cuases for such poor operation are as follows:
(1) the assembly consisting of the jack, the slider, the fork, and the dog-clutch is the seat of substantial solid frictions. Such frictions become much reduced when the motion is being started;
(2) any slowing down in the rise of pressure in the jack brings about a delay between the advent of the command of execution and the meshing of the synchromesh device. This time is included in the total time for the gear change concerned, but is completely useless since it does not correspond to any effective synchronization action. This shows that using a supply jet influences the system adversely;
(3) supplying the jack under full pressure allows overcoming the solid frictions readily. But said frictions decrease when a displacement takes place, so that the beginning of the synchronization occurs with a substantial stress, and thus with a very substantial synchronization torque at a high differential speed. This results in a very high required instantaneous power which should be avoided (unduly high surface temperature on the synchromesh cones, which leads to a premature destruction of the latter).
An object of the present invention is to obviate the above-mentioned drawbacks by getting the various synchronization steps under control.